I run a X5690 flat out for rendering purposes in a luggable Clevo X7200 system.
Runs on scary high temperatures 90-95° (TJ max = 101°), but with airco turned max it mantains that temperature even at 100 hour sessions.
Quoting on new project now, that would require 2000 hours of rendering on single X5690.
I'm seriously considering buying new watercooled system with dual Xeon E5-2687W processors.
I was hoping to overclock all 16 cores to 4 GHz (would still be 500 rendering hours).

DO I UNDERSTAND CORRECTLY THAT ONLY 1 CORE OF EACH PROCESSOR CAN REACH 4 GHz ?
Could a Core i7-3960X be clocked higher on all cores simultaneously ?
But that i7 has only 6 cores, and on http://www.cpubenchmark.net/overclocked_cpus.html
the overclocked i7 shows the same score as a standard Xeon E5-2687W.
THAT WOULD MEAN THAT OVERCLOCKING AND WATERCOOLING OFFERS NO ADVANTAGE ON XEON's !?

You cannot overclock SB-EP Xeons to 4ghz with all cores at 100% load. With all cores fully loaded the maximum increase will be a few % because the only thing you will be able to adjust is the BCLK. Higher multis will not work and the BCLK-straps will not work.

A highly overclocked 3930K/3960X will clearly beat a single E5-2687W (cinebench score for the i7 @ close to 5ghz is close to 15 and around 13 for the Xeon). But two E5-2687Ws of course will with no trouble beat the normal i7 setup at around 25.5 points or so.

Ok thanks for the quick raection. But would it make sense to run two highly overclocked 3930K/3960X on all 12 cores simultaneously at 100% ?http://www.cpubenchmark.net/multi_cpu.html does mention running two 3960X as nr 3 score.
Strange enough that shows a dual E5-2687W set up on 4 with almost same score.
What I don't understand is that a dual E5-2687W has more or less doubled its single score, but the dual 3960X comes in at 2.3 its single score.
Would the dual i7 be running overclocked on all 12 cores, while the E5 runs standard on all 16 cores ?
I guess why border voiding warranty when standard E5 can match overclocked i7 !?

First of all I wouldn't trust passmark since the results there are taken from so many systems that they're not really reliable. The changes between different systems and the way they've been clocked, are used, cooled, what programs people are running, OSes, etc. make the scores somewhat bad. If we're talking about rendering I would mostly look at cinebench scores from different reviews to make comparisons.

Anyways, one thing you seem to miss is that you can't use more than a single 3930K/3960X in a single system. The consumer i7s only have one QPI link and thus do not work in multi CPU configurations. The Xeons will work in 2 CPU configurations since they have dual QPI links. Your options are basically a single i7, a single Xeon or dual Xeons.

So yes, if you wish to have the absolute best rendering performance and lowest render times the dual E5-2687W system is the best option by a really big margin. It will be expensive obviously but I'm assuming the budget would allow for that.